1. Field of the Invention
The present invention relates to a method of manufacturing single-crystal silicon by the Czochralski pulling method (hereinafter called the CZ method).
2. Description of the Background
Single-crystal silicon is an essential element in the manufacture of various semiconductors and highly effective solar batteries and can be obtained industrially by the CZ method, wherein the single-crystal silicon is pulled up from a silicon melt. Another known method of manufacture is the floating zone method (the FZ method), in which a polycrystalline silicon rod is inserted into an induction coil to create single-crystal silicon. The CZ method is regarded as economical, because it requires less processing and because it provides a more profitable yield than the FZ method. However, the CZ method does not achieve single-crystal silicon of a quality equivalent to that obtained by the FZ method, mainly because of the fact that the resultant single-crystal silicon is contaminated by impurities from the worn quartz crucible containing the silicon melt.
Also, the continuous pulling of a single-crystal silicon rod for hours is an inherent feature of the CZ method, which is difficult to do for long periods of time. In principle it is possible to make the specific resistance of the single crystalline silicon rod constant by continuously drawing up the Si melt for a long time which results in substantially improved yield and greater efficiency of furnace operation. However, no fully satisfactory continuous pulling operation is known in achieving these objectives primarily because of wear of the quartz crucible similar to deterioration of the quality of single-crystal silicon.
The introduction of an electromagnetic melting step into the CZ method has been recently watched with keen interest in light of the above objectives. The electromagnetic melting process is a kind of floating melting method, which uses a circumferentially divided watercooled conductive crucible for melting the material placed therein as it floats through an electromagnetic field. The principle has been adopted in a polycrystalline silicon manufacturing apparatus as disclosed in Japanese Unexamined Patent Application Nos. 61-52962, 1-264920 and 2-30698 and the like. The electromagnetic melting process may be employed in the CZ method thereby allowing a silicon melt placed in a crucible to be kept away from the inner surfaces of the crucible, in order to substantially reduce wear and consumption of the crucible in principle, thereby preventing the single-crystal silicon from being polluted by impurities from the crucible which ensures that the resultant single-crystal silicon has a quality equivalent to that obtained by the FZ method and which enables continuous pulling of single-crystal silicon for extended periods of time.
The above technique, however, has many problems and has not yet been practically used on an industrial basis. One of the factors hindering industrialization of the CZ method using the electromagnetic melting process is an agitation or stirring phenomenon of the silicon melt which is in the crucible, as the melt is kept away from the inner surface of the crucible. This stirring phenomenon is shown in FIG. 6.
FIG. 6 shows an electrically conductive crucible 2 divided in a circumferential direction and arranged inside an induction coil 1. The respective divided pieces of crucible 2 are electrified by an alternating current flowing through induction coil 1 and thus a magnetic field is applied to silicon melt 3 within crucible 2 in the longitudinal axial direction. As a result, silicon melt 3 is electrified in a circumferential direction which generates Joule's heat. However, this magnetic field is inevitably weakened in the upward and downward directions because of the construction of the induction coil 1 and a stirring force is added to silicon melt 3 by a difference of Lorentz force on the basis of such an uneven distribution of the magnetic field. Further, the flow of liquid by this stirring force is remarkably intensified by the accumulative effect of the following three phenomena.
The first effect is the strong stirring force imparted to silicon melt 3 which is proportional to the power applied to the induction coil 1.
The second effect is the stirring force which is increased in the low-frequency band. That is to say, in the electro-magnetic melting of silicon, if the induction frequency is high, the current efficiency is improved but the skin effect is quite noticeable, so that eddy currents do not arrive at the central portion of the silicon melt 3 within the crucible 2 and thus the central portion of silicon melt 3 is not sufficiently melted. Accordingly, during the production of a large-sized single crystalline rod 4 the induction frequency is inevitably reduced. For example, a low-frequency band of 2 to 3 kHz is selected. However, at such a low-frequency band, the electric current penetrates to a depth of silicon melt 3 within the crucible 2 and thereby generates a strong stirring force within silicon melt 3. This is a substantial disadvantage in the production of large-sized single crystalline rods 4 these days.
The third effect is that the silicon melt floats within crucible 2 and is apt to move itself.
The flow of liquid by the stirring force is intensified incomparably and augmented by thermal convection in the melt which is shown by the broken line in FIG. 6. Thermal convection is generated by conventional melting in the crucible. As a result, the liquid surface of the silicon melt 3 violently surges and thus swirl defects, transfers and the like resulting from temperature fluctuations are produced in the single crystalline rod 4 drawn up from the silicon melt 3. All of this results in deterioration in quality of the single crystalline rod 4.
Another factor which hinders industrialization of the CZ method using the electro-magnetic process is the increased cost of electric power. That is to say, in the CZ method which uses the conventional electro-magnetic melting process, almost half of the electric power applied is consumed in the heating of the crucible. Accordingly, in spite of direct heating of the silicon by eddy currents, the economy of the process is not improved. A need therefore continues to exist for improvements in the CZ method of forming single crystal silicon.